Somewhere in Boston there’s at least one refrigerator with a little news clipping posted on it. The story was published on boston.com on May 6, 2011, and announced the 45 winners of the city’s annual scholarship program for outstanding seniors. Third to last on the alphabetical list was Dzhokhar Tsarnaev, then 17 and a student Cambridge Rindge & Latin School, who would be receiving a check for $2,500.

“My son is a true angel,” said his father Anzor Tsarnaev when he was reached by the Associated Press at his home in the Russian republic of Dagestan on Friday. “He is such an intelligent boy. We expected him to come on holidays here.”

It turns out, of course, that Dzhokhar, the surviving suspect in the Boston bombings, wasn’t an angel. And nor was his brother and accomplice Tamerlan, who was killed in a shootout with police today. But in the case of Tamerlan, that was already clear. An amateur boxer, he was arrested in 2009 on a domestic-violence charge and was quoted as saying in a photo essay, “I don’t have a single American friend. I don’t understand them.” Dzhokhar had so far seemed to be nothing but a well-adjusted kid.

So the cause and effect is easy to assume: good boy Dzhokhar, now 19, came under the malign influence of bad boy Tamerlan, 26, and the result was tragedy. Maybe. The science of siblings is a rapidly growing field in developmental psychology, one that’s built up an impressive body of research over the past 20 years. Still, it’s not equipped to reckon with any certainty with a crime of this magnitude, and it never really has been — which partly explains our enduring fascination with criminal siblings like the Menendez brothers, who killed their parents in Beverly Hills in 1989, or the legendary James brothers of the 19th century. But the data that exist so far do provide a starting point.

Siblings have historically been viral vectors for one another’s risky behaviors. A girl whose older sister is a teen mom is six times more likely to become one herself. Alcohol consumption is twice as likely among kids with at least one sibling who already drinks; for smoking, the risk increases fourfold.

In the case of criminality, the numbers are all over the map, in part because so many other X factors are involved — income, education, the presence of one parent vs. two in the home. So too does sibling-on-sibling violence. Numerous sibling studies have shown that when brothers — who are typically of far greater interest to researchers studying criminality since they offend much more often than girls do — make a habit of settling their differences with one another by coming to blows, they are far more likely to become violent offenders as teenagers and adults. One University of Florida study of 538 college students found that the same boys, steeped in brother-on-brother violence, were also likelier to commit sexual abuse or battery.

But those are just numbers, which mean both a great deal and nothing at all when it comes to determining why any sibling pair will commit a collaborative crime. In the current case, there were a few factors that probably nudged those risk figures higher. For one thing, the young men lived together. Proximity is a critical element in what psychologists call “delinquency training” among siblings. This is true for lower-grade risk behaviors like smoking or drinking, since a younger sibling, who typically picks up the habit later than an older one, must be able to observe and model the bad behavior — to say nothing of getting hold of the forbidden substances in the first place.

Psychologist Elizabeth Stormshak of the University of Oregon conducted a study in which sibling pairs were interviewed on videotape along with other kids and were asked to pretend they were planning a party and that some kids were going to bring drugs. They were then told to debate whether that was alright and whether they might want to try them themselves. In general, the likelihood of a younger sibling’s saying that that was indeed O.K. was directly linked to whether an older sibling had already said the same. The direct mimicry would not have been possible without direct proximity.

It’s a lot easier to pick up a cigarette or a pill than to pick up criminality, of course, but when adult siblings share a home, the influence one exerts on another can be hothoused. Researchers believe that siblings who live together as young adults — which is often a sensible and perfectly healthy arrangement, particularly in tough economic times — will ultimately hit a developmental ceiling at which their continuing total-immersion contact with a member of their original nuclear family begins to stunt their social growth. Consciously aware of that or not, they tend to separate and enter what psychologists call a sibling-moratorium phase, spending far less time with each other until they’ve settled into their own adult lives. Hanging around together too long prevents that developmental step.

“The moratorium tends to happen when siblings are in their 20s,” psychologist Katherine Conger of the University of California, Davis, told me in interviews for my book The Sibling Effect. “The sibling relationship must recede for a while because they are working on [their own] issues.” Psychologist Victoria Bedford of the University of Indianapolis calls the same phenomenon “the hourglass effect,” with the choke point in the middle of the glass representing the time of minimum sibling contact.

Few siblings who cohabit longer than is good for them become criminals, of course. In the case of the Tsarnaev brothers, one exacerbating factor might have been cultural isolation — living in a country that is increasingly nativist. But the record doesn’t support that. The family emigrated to the U.S. in 2002, and both Dzhokhar and Tamerlan seemed to have assimilated reasonably well.

“He was normal,” one of Dzhokhar’s former classmates at Rindge & Latin told the Boston Globe. “He kind of fit in with everyone.”

“He was gracious,” said a neighbor in an interview with the New York Times. “He told me he was from Chechnya, and I asked him what that was like and he never expressed any bitterness toward his situation.”

There have been fewer kind words spoken about Tamerlan. In addition to his domestic-violence charge, he recently showed up at a mixed-martial-arts gym that he hadn’t visited in years and began acting out — using other people’s equipment, walking on mats with his shoes. “It was a clear indication that something was up,” one witness told the Times. “It was completely out of place for him.”

That comparatively minor misbehavior could offer a flicker of insight into the brothers’ crime if it was suggestive that a deeper sociopathic fire was burning. Typically, the real variable in criminal collaboration is not so much the blood relationship of the perps, but the interpersonal relationship — the influence of a compelling, charismatic personality on another, weaker one. In those cases, the fact that they’re siblings may be a secondary variable — simply the means by which the two people came to know each other and remain fixtures in one another’s lives, but little more.

That weak-strong push-pull has been a factor in some of the country’s worst joint crimes, committed by people who were not kin at all. Eric Harris was clearly the leader of the Columbine pair — pathologically charming, especially compared with his quieter, less confident accomplice, Dylan Klebold.

“Eric was an incredible individualist,” Brooks Brown, a schoolmate who knew both boys, told TIME in 1999. “Charismatic, an eloquent speaker, well-read, the kind of guy who could bulls–t for hours about anything and be witty and brilliant.” Klebold, according to classmates, teachers and the chilling videotape the boys left behind, was clearly the follower.

Something starkly similar defined the relationship between Lee Boyd Malvo and John Allen Muhammad, the snipers who killed 10 people and wounded three others in a shooting spree that spanned three weeks in October 2002 in Washington, D.C.; Maryland; and Virginia. Malvo was 17 at the time of the crimes, Muhammad was 42. The two had met through family in Antigua, and before long Malvo fell wholly under Muhammad’s influence. FBI agent Brad Garrett, who was among the first to interview Malvo in 2002, said it was clear the boy was “under the spell of Muhammad.” Last year, in an extensive interview with the Washington Post, Malvo — who is serving multiple life sentences with no hope of parole and thus has little motivation to try to clean up his rep today — confirmed as much.

“He understood exactly how to motivate me by giving approval or denying approval. It’s very subtle. It wasn’t violent at all,” he said. “He picked me because he knew he could mold me. He knew I could be what he needed me to be … He could not have chosen a better child.”

That pattern may have held in a subtler way for Lyle and Erik Menendez. Lyle, older by two years, was said by friends to have been the wittier and more personable of the two. Erik, like Klebold, was much quieter. This could even have been true to some extent of the legendary James brothers. Both Jesse and older brother Frank were outlaws of the first order, their talent for violence practiced in the Civil War and the guerrilla actions that preceded it in their home state of Missouri. But Jesse was the undeniable leader of the criminal enterprise known as the James-Younger gang, and after he was killed in 1882 by a member of the group hoping to claim the advertised reward, the onetime partners scattered. Frank was later tried and acquitted and spent the remaining 30 years of his life working variously as a shoe salesman and a popular speaker on the lecture circuit.

Psychic forensics following any crime will always be an imperfect art. You can’t dust for emotional influence; there is no DNA test for bloodlust. This is especially true when one or more of the participants in a crime does not survive the manhunt that follows. More and more bits of the Tsarnaev brothers’ history will come together in the weeks and months of investigation that are sure to follow the siege in Boston. The sibling bond, which can be such a powerfully good thing, was surely a powerfully bad one in this case. Whether it was the determining thing is, and may always be, impossible to say.